''How would you feel if someone said to you we're going to take 20 per cent of your income away overnight?'' says Justin Goldsmith from Corkscrew Cellars.

The co-owner of the Oxford Street bottleshop expects to find out in just a few hours.

The Sydney Gay and Lesbian Mardi Gras parade is the biggest night of the year for many businesses along the entertainment strip. And like many in the business of alcohol, Corkscrew Cellars' night is ending early.

New restrictions that came into force on Sunday will require it to close at 10pm - about an hour before the end of the parade that will draw thousands of revellers past the shop.

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It also marks the first Saturday, as well as the first Mardi Gras, that nearby pubs and clubs will not be permitted to admit any new patrons by 1.30am, or serve alcohol after 3am.

''It is our Christmas,'' said Mr Goldsmith's business partner, Valentina McMahon. ''We're really worried about the loss of trade.''

By the time Corkscrew Cellars would typically close at midnight on Mardi Gras, it would have sold as much as it otherwise would in a whole week, Ms McMahon said - sales spurred on by post-parade purchasers keen to grab a bottle to take home or a case to bring to an after-party.

John Cassimatis, co-owner of Liquor On Oxford, said he didn't see what liquor stores had to do with violence.

''We haven't seen violence in this store or around us in the eight years we've been here,'' Mr Cassimatis said.

It was a view rejected by Michael Thorn, chief executive of the Foundation for Alcohol Research and Education, who said package liquor contributed to street drinking and alcohol-related harm.

''If the new measures are strictly enforced we should see an immediate reduction in emergency department presentations and assaults or arrests,'' he said.

The measures could cost The Oxford Hotel between $30,000 and $40,000 in revenue on its biggest night of the year, its owner Jaz Mooney said.

''As business owners and hotel operators, we understand that this is new ground, but the double whammy for us is that it's just happening before Mardi Gras,'' he said.

Sydney lord mayor Clover Moore described the timing of the new measures as ''disappointing'' and called for some Oxford Street venues to be granted a one-off exemption.

But NSW Premier Barry O'Farrell said he made ''no apologies for these tough measures''.